Tuberculosis



Last Review Date: August 14, 2017


What is it?

Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis. It primarily affects the lungs (pulmonary TB), however, other parts of the body may be involved. Worldwide, TB is still the leading cause of death due to infection, killing more than 1 million people a year. Most of these deaths are in people living in poor countries and the majority of these deaths are preventable. It is spread through the air from person to person through lung secretions such as sputum (spit or phlegm) or aerosols released by coughing, sneezing, laughing or singing. Most of those who become infected with M. tuberculosis manage to confine the microorganisms to a few cells in their body, where they stay alive in an inactive form. This inactive or latent TB infection does not make the patient sick or infectious and, in most cases, it does not progress to cause active or symptomatic tuberculosis. However, some patients - especially those with damaged or compromised immune systems - may proceed directly from initial TB infection to active tuberculosis. Approximately 10% of infected people develop active TB, with the greatest risk of disease occurring in the first 2 years after infection.

TB is not a common disease in Australia and the incidence of TB in Australia has remained stable since 1985, at approximately 5.4 cases per 100,000 population. The risk of infection is higher in migrants from high incidence countries and indigenous Australians at 21.7 and 8.1 cases per 100,000 population, respectively.


Signs and symptoms

Latent TB infection does not cause any symptoms. Someone may have latent TB infection for years without knowing it. The symptoms of active tuberculosis depend on what part(s) of the body are involved. The most common symptoms are pulmonary and include chronic cough, haemoptysis (coughing up sputum containing blood), fever, weakness, night sweats and weight loss.

If the TB is extrapulmonary (outside of the lungs), it may cause few noticeable symptoms or a wide range including:

  • Back pain and paralysis (spinal TB)
  • Pain associated with reproductive system or urinary tract, possibly resulting in infertility
  • Abdominal pain
  • Enlarged lymph nodes
  • Altered mental state, headache and, in extreme cases, coma (TB in the brain and/or central nervous system)
  • Disseminated/miliary TB (i.e. spread throughout the whole body) can cause a range of the above symptoms

All of these symptoms may also be seen in a variety of other conditions. A definitive diagnosis of active tuberculosis depends on the positive identification of Mycobacterium tuberculosis organism in the body fluids or tissues. 


Tests

Latent TB infection

Latent TB is usually diagnosed by a positive TB skin test (TST) and a whole blood test called Interferon-Gamma Release Assay (IGRA). These are used for patients who are at a high risk of contracting the disease and for those who work or live with high-risk patients. The TB skin test may also be done as part of a physical examination prior to starting school or a new job. Positive results may indicate a latent TB infection or previous BCG vaccination and should be followed by a clinical assessment.

In Australia, TST is a Mantoux type skin injection of tuberculin that relies on detecting delayed type hypersensitivity at 48-72 hours.

IGRA requires approximately 3 mL of blood to be collected into three separate tubes which should be promptly transported to a laboratory and incubated at 37°C within 16 hours of collection. IGRA and TST have equal diagnostic sensitivity but IGRA can distinguish between infection with Mycobacterium tuberculosis and BCG vaccination.

Active tuberculosis

To diagnose active TB of the respiratory tract, at least 2-3 sputum specimens are collected for microscopy and culture. The best samples are obtained first thing in the morning when sputum is most likely to contain the most TB bacteria. Serial sputum samples increase the yield. Approximately 85, 10 and 5% of all cases detected by three serial examinations are found on the first, second and third sputum examination, respectively.

If extrapulmonary TB is suspected, samples are collected based upon where in the body the infection is likely to be. Multiple samples of gastric washings/aspirates or urine may be collected and submitted to the laboratory. Sometimes cerebral spinal fluid (CSF), biopsied tissue, or other body fluids are also collected.

A presumptive diagnosis of TB can be made by examining a smear of the patient's specimen under the microscope after it has been stained with a special stain to detect acid fast bacteria or AFB. Although simple and quick, microscopy has an overall sensitivity of only 30-80% and is less useful for TB disease in children or immunocompromised patients and in extrapulmonary TB. Positive microscopy results are often reported in the WHO scale:

Microscopic detection of TB: staining for acid fast bacteria (AFB)
‘+’ indicates presence of AFB
Report (WHO scale) AFB detected in at least 100 high power microscopy fields (HPFs)
Negative No AFB detected
Occasional 1-9 AFB per 100 HPFs
+ 10-99 AFB per 100 HPFs
++ 1-10 AFB per field in at least 50 HPFs
+++ more than 10 AFB per field in at least 20 HPFs.

Positive AFB smears cannot distinguish between the different species of ‘acid-fast’ bacilli and AFB cultures are set up on all samples submitted for microscopy for AFB. Nutrients and incubation provide a supportive environment for the slow growing mycobacteria. The results of cultures are definitive: they can tell your doctor what organisms are present and what drugs are likely to kill them but they take time - usually 2-3 weeks (but up to 8 weeks) for positive samples. 

PCR tests are available for TB and can be performed on a wide range of sample types. PCR tests give a more rapid result than culture, however are not as sensitive and cannot not provide the detailed information available from culture. PCR testing for Mtb performed directly on patient samples is currently used as an adjunct to microscopy, culture and susceptibility testing. 

Once M. tuberculosis has been identified and treatment has begun, AFB smears and cultures are used to monitor the effectiveness of treatment.

Non-laboratory tests

X-rays are often used as a follow-up to positive TB skin tests to look for signs of mycobacteria growth and to help determine whether someone has active tuberculosis or a latent TB infection. Infection with TB can cause a number of characteristic findings on x-rays, including cavities (holes) and calcification in organs such as the lungs and kidneys.


Prevention

Prevention of TB infection lies primarily in identifying, isolating, and treating those who have it before they pass it on to others.

A vaccine called BCG (bacillus Calmette-Guerin) is often routinely administered in parts of the world where TB is much more common, however this vaccine is not completely effective and only prevents the severe forms of TB such as military TB and TB meningitis and is most beneficial when given in infancy.

The Australian Immunisation Handbook 10th edition, 2017 immunisation programme does not recommend vaccination with BCG for the general population. Special risk groups where BCG vaccination should be considered include:

  • Aboriginal and Torres Strait Island neonates living in regions of high TB incidence
  • neonates born to parents with leprosy or a family history of leprosy
  • healthcare workers who are likely to encounter people with TB or those involved in conducting autopsies
  • In children who will be travelling to live in countries of high TB prevalence (WHO defines ‘high-risk’ countries as those with an annual incidence of TB of greater than 40 per 100,000 population). BCG may be beneficial, especially in children <5 years and should be discussed with local state/territory TB services or with a paediatric infectious diseases specialist

Tuberculosis is a notifiable disease in Australia and clinicians and laboratories must report all cases to Public Health services.


Treatment 

Early Detection
Early detection depends on identifying those at risk and testing them at regular intervals for latent TB infection. It also depends on recognising, diagnosing, and treating those who progress to active tuberculosis. 

Latent Tuberculosis
The decision to treat latent TB infection is up to you and your doctor. If follow-up testing reveals no indication of active tuberculosis and you are not considered at a high risk for developing active TB, your doctor may decide to simply monitor your health at regular intervals (since about 90% of those with latent infections never develop active tuberculosis). 

If, however, your doctor is of the opinion that you are at risk of developing active TB, you may be treated with a six month course of an antibiotic called isoniazid. It is necessary to take it for the entire treatment period to ensure that all of the latent bacteria have been killed.  Failure to do so can result in treatment failure and development of multi-drug resistant TB which can prove extremely difficult to treat. Your doctor may use lab tests to monitor your liver during this time period, as isoniazid can sometimes affect how the liver functions. 

Active Tuberculosis
Active tuberculosis must always be treated. Once M. tuberculosis has been positively identified, your doctor will start you on a treatment program that involves taking several drugs for several months. The length of treatment depends on the results of the AFB smears and cultures used to monitor the effectiveness of treatment. 

Although your symptoms will often go away after several weeks it is crucial that you continue to take your drugs for the entire time period. There are a large number of mycobacteria to kill and it takes several months to make sure that all of them have been eradicated. If treatment is not continued, the TB can come back, and this time it may be more difficult to treat. It may now be resistant to the first choice drugs, requiring treatment for several more months with drugs that have more side effects. 

Those with active tuberculosis may be encouraged to participate in DOT (directly observed therapy). This involves taking your medication each day, or several days a week, under the supervision of medical personnel. This increases patient compliance with treatment and decreases the number of people that have to be treated again because their TB has returned.
 


Related Pages

On this site
Tests: Tuberculosis screening tests, Acid fast bacilli culture
Conditions: HIV
Screening: children, young adults, adults, adults 50+

Elsewhere on the web
Better Health Channel: Tuberculosis (TB)
healthdirect Australia: Tuberculosis
World Health Organization: Tuberculosis (TB)
Australian Immunisation Handbook 10th edition, 2017